This invention relates to improvements in vapor-liquid contact apparatus used in chemical processing, and particularly to the type of such apparatus in which a vessel contains a plurality of substantially horizontal trays which support a vapor-liquid mixture; liquid is introduced at the upper end of the vessel and it flows down from tray-to-tray, via downcomers; each downcomer has a floor which form a downwardly facing outlet for releasing liquid from the downcomer to a subsequent tray; and, the trays are apertured to provide bubbling areas through which ascending vapors can rise to contact liquid and/or vapor-liquid mixtures which are supported on and flowing across them.
It has been recognized in the art that the performance of contact tray apparatus can be enhanced if the liquid flow on the tray is uniform in the respect that the flow in the lateral areas of the tray is substantially the same as the flow along the central flow axis thereof. Heretofore, shaped downcomer tips, directional vapor outlets and other means have been utilized for this purpose.
It has also been recognized in the art that, by providing apertured floors in downcomers, the deentrainment of vapors from liquid from the vapor-liquid mixture in downcomers can be improved by providing the downcomers with apertured floors which increase the residence time of the vapor-liquid mixture in the downcomers. Downcomer floor configurations disclosed in this specification serves both to increase the residence time of the vapor-liquid mixture in the downcomer and to provide a more uniform flow of the vapor-liquid mixture in the bubbling area of a tray.
According to the present invention, the flow of fluid into the bubbling area of a tray is made more uniform across the width of the bubbling area by providing a novel shape and location of a downwardly facing liquid outlet opening of a downcomcr which feeds the tray. Utilizing the simple and effective design of the present invention, the apparatus will have increased capacity, higher efficiency, and a greater operating range. The risk of fouling or plugging the downcomer outlet is minimized. By providing adjustability of the components which define the novel downcomer outlet opening, the apparatus can be fine-tuned to achieve optimum performance.
It has been observed experimentally that, when liquid is released through a circular outlet opening (a pour point) in a downcomer floor, and the liquid from the pour point strikes a target area on an unobstructed flat surface, the liquid flows from the pour point in the form of an expanding shallow inner ring which extends radially from the pour point. The liquid depth suddenly increases in an annular area, referred to in this specification as an "hydraulic jump ring" which is concentric with the target area and the pour point.
When two pour points are close enough together that the hydraulic jump rings emanating from their respective target areas interact, an even deeper wave is formed equidistantly from the target areas. By using an elongated downcomer outlet opening or openings in accordance with the present invention, these deeper waves are eliminated or reduced in number.
When the target area of a pour point is near an obstruction such as the inner wall of a vessel, the liquid emanating radially from the target area will strike and rebound from the vessel wall. Due to the concave curvature of the wall of a cylindrical vessel, some of the rebounding liquid will be directed toward the central flow axis of the tray, thus causing a focusing effect which results in a higher flow rate at the central flow axis of the tray than at the sides thereof. This is of particular concern when the outer edge of a downcomer outlet slot is contiguous with the inner wall of the vessel, thus avoiding the undesirable rebound and focusing effect which is associated with prior floored downcomers.
This invention is based in part on the recognition that there are inherent deficiencies in the liquid distribution at the upstream ends of the bubbling areas of the trays in the apparatus described above, particularly due to the configurations and arrangements of the apertures in the downcomer floors. Significant features of the apparatus disclosed in this specification are believed to represent new and beneficial approaches to the design and construction of such apparatus.